Modern commercial transport aircraft are provided with a multitude of safety features, including fire suppression systems. Fire suppression systems are typically placed in high temperature areas of the aircraft, including the main engine compartments and the auxiliary power unit compartments. The fire suppression systems are then activated manually or automatically when a compartment fire is detected. Accordingly, the aircraft is typically outfitted with bottles or other reservoirs of fire suppressants, which are discharged in the event of a fire and then recharged after the aircraft lands.
Aircraft designers are tasked with designing fire suppression systems that are effective and yet lightweight, so as to provide in-flight safety without unnecessarily sacrificing payload weight and/or fuel efficiency. The weight of an aircraft fire suppression system depends to a large degree on the volume of fire suppressant that the system must carry. The effectiveness of the system depends on how quickly the system is able to establish a selected concentration of the fire suppressant throughout the compartment in which the suppressant is discharged. Designers typically estimate the amount of fire suppressant required for a particular aircraft installation, and then validate the estimate via experimental techniques. A typical experiment includes outfitting a representative compartment volume with concentration detectors, injecting a fire suppressant into the vented compartment volume, and experimentally determining whether the concentration of the fire suppressant in the compartment remains high enough for long enough in a large enough volume of the compartment to suppress a compartment fire. This estimation/validation process is repeated until a lightweight, effective design is produced.
Estimating the manner in which the fire suppressant is dispersed in the compartment is a difficult task because, among other factors, the suppressant is a mixture of multiple constituents in both liquid and gas phases, the compartment geometry is complex, the flow in the compartment is three dimensional, and the flow is unsteady (e.g., it varies with time). Accordingly, designers have increasingly relied on computer-based computational fluid dynamic (CFD) methods to more accurately simulate the flow of fluids in confined volumes. Commercially available CFD codes are capable of simulating the flow of multi-phase fire suppression mixtures through pipes and ducts (such codes include Hflow, available from Lassalle Technologies at www.lassalle.com), and simulating the dispersion of suppressants within an engine compartment (such codes include Fluent, available from Fluent Inc. of Lebanon, N.H.).
One drawback associated with the foregoing techniques is that it has been difficult to accurately simulate the flow of fire suppressant liquid droplets into an engine compartment volume. As a result, the accuracy with which current techniques simulate the time-varying concentration of the fire suppressant within an engine compartment volume may be significantly limited. Accordingly, the designer must often rely on repeated and expensive experiments before validating the effectiveness of a fire suppression system design.